suffixes - What is 方 used for (when attached to a た-verb)?

I have the following sentence:

• 早く行った方が良かったでしょう。

Here, if we take it apart we shall have:

• 早く(Adverb) 行った(Verb, Past tense) 方(?) が(GA, Subject particle) 良かったでしょう(Verb, Past tense) 。

What is the function of 方 in this position? I've read that it can be suffix, suffix of what?

Answer

A方{ほう}がB means "more B if A" or "Ber if A":

早く行った方が良かったでしょう。
It would have been better [more good] if (we/you/I etc) had gone early, would it not?

The 方 here indicates a direction/side when comparing 2 or more things ([3] （イ） of this definition at Daijirin), in this case implying going early would have been better than going later.